Infection of mice by Borrelia burgdorferi can be prevented by the prior immunization of the animal with outer surface protein (Osp) A and OspB. Protection cannot readily be afforded, however, by immunization with these proteins post infection, and animals and humans frequently remain chronically infected by Borrelia for years unless treated. This, and several lines of evidence suggest that changes in borrelial gene expression occur at different stages of its life cycle in the tick and the mammalian host. These changes are likely to be of importance for mechanisms of pathogenesis and immune evasion. We will utilize a variety of approaches to identify proteins that are differentially expressed in vector and host, to clone and express the genes that encode these proteins, and to assess the role of these proteins in protective immunity. These proteins will be provided to Barthold and Malawista for pathogenesis studies in their proposals, and to Fikrig for studies of immune selection. We will address the following specific aims. First we will demonstrate the antigenic differences between Borrelia in mammalian host, culture, and ticks by generating antibodies against proteins expressed specifically in ticks and infected mice. Second we will isolate borrelial genes differentially expressed in ticks, in BSK culture, or in the mammalian host in vivo by using immunologic screening and differential hybridization techniques. Such genes obtained will be characterized, their DNA sequences determined, and expression patterns validated. The proteins, and the immune response for those proteins, that are expressed differentially in B. burgdorferi derived from ticks, or culture in vitro, or the mammalian host, will then be characterized. The DNA sequence of these genes will be determined in Borrelia of diverse origin to assess the level of sequence conservation. Finally, the role of proteins differentially expressed in tick and host will be assessed in protection from infection by active and passive immunization of C3H mice with these proteins, followed by infection of these mice with either tick or syringe challenge, initially with B. burgdorferi sensu stricto, and if protection is afforded, subsequently with B. afzelii and B. garinii strains.